Journal of Pressure Equipment and Systems 3 (2005) 157-161

Experimental study of fatigue crack growth rates of 17-4PH, 1Cr13 and 2Cr13 in air and chloride conditions

Junwen Hao *, Shihua Tang, Hongliang Pan, Jin Chen

School of Mechanical and Power Engineering, East China University of Science and Technology, Shanghai 200237, China

Abstract

The shafts made of 17-4PH in extruder dryer often lose their serviceability under combined effects of both corrosion and repeated stresses. In this paper, in order to find out an alternative material which has a good resistance to corrosive fatigue damage, the non-symmetric cyclic experiments with r=0 were carried out on three kinds of materials of 17-4PH, 1Cr13 and 2Cr13 in laboratory air and chloride environments at room temperature. The research work has demonstrated that the fatigue crack propagation is slower in chloride solution than that in air environment. Comparisons of fatigue crack growth rates among these three kinds of materials have suggested that 17-4Ph is susceptible to corrosion damage in an aggressive environment. However, the stainless steel 2Cr13 has a better resistance to the corrosive fatigue damage and can be used as an alternative material working under the combined action of an alter-nating stress and a corrosive environment.

Keywords: Corrosion fatigue; air; Cl-; Crack growth; 17-4PH; 1Cr13; 2Cr13.

1. Introduction

Corrosion fatigue, which usually reduces fatigue

strength considerably down to 75 to 25% of the fatigue strength in dry air, is the result of the combined action of an alternating stress and a corrosive environment [1-4]. Like ordinary fatigue cracks, corrosion fatigue cracks generally propagate at right angles to the maximum tensile stress in the affected region. How-ever, cracks resulting from simultaneous corrosion and alternating stress often propagate much more rapidly than cracks caused by alternating stress alone. Also, corrosion fatigue failure usually involves several par-allel transgranular cracks. Since no metal is immune from some reduction of its resistance to cyclic stress-ing if the metal is in a corrosive environment, the cor-rosive fatigue should be taken into account in design of components. Control of corrosion fatigue can be ac-complished by either lowering the cyclic stresses or by corrosion control [5].

The shaft used in extruder dryer in petrochemical industry is a kind of cantilever shaft runs at the speed of 220-280 rpm. In general, such shafts run with a steady torsional load superimposed with bending stress either due to the shaft’s self-weight or possible mis-alignment between the bearings [6]. In some cases, such shafts in petrochemical industry also severely suffer from corrosion fatigue problem, in which corro-sive effects of environment and cyclic mechanical * Corresponding author. E-mail address: jwhao@ecust.edu.cn (J. W. Hao).

loading degrade the material properties. Reduction in fatigue properties is greater from the combined effect of both corrosion and repeated stresses acting on to-gether than those factors acting separately [7]. Ac-cording to information given by the user, such a shaft made of 17-4PH used in extruder dryer failed after 6 months of service. The metallographic examinations have suggested that the shaft was corrosive induced fatigue failure [8, 9].

As one in a series study on the failure analysis of chemical equipments, in this paper, in order to find out an alternative material which can be used effectively in corrosive environment, the non-symmetric cyclic ex-periments with r=0 were performed on materials of 17-4PH, 1Cr13 and 2Cr13 in laboratory air and chlo-ride environments at room temperature. In the experi-ments, fatigue crack growth rates of these three kinds of materials were investigated in both air and corrosive environments. Comparisons of fatigue crack growth rates of 17-4PH, 1Cr13 and 2Cr13 were also made in order to find what kind of material has a good resis-tance to the corrosive fatigue damage.

2. Experimental method

2.1. Test specimen

In this work, specimen made of 17-4PH was cut from the broken shaft directly. The chemical composi-tion of 17-4PH is given in Table 1. 1Cr13 and 2Cr13 specimens were made of new commercial materials. The chemical compositions of 1Cr13 and 2Cr13 are given in Tables 2 and 3, respectively. The dimension of test specimen used in the experiment is shown in Fig. 1

J. W. Hao et al. / Comparison of fatigue crack growth rates of 17-4PH, 1Cr13 and 2Cr13 in chloride environment 158

(plate specimen with thickness of 10 mm), where circle shaped notch is used to fix the fatigue crack position. The specimens were polished elaborately using lubri-cant before experiment in order to observe the fatigue cracks clearly via microscope or scanning electron mi-croscope (SEM) during experiment.

2.2. Test equipment

The present study principally concentrates on the corrosive fatigue behaviour of a rotating suspended shaft working in rubber extruder dryer. In order to simulate the practical working condition of such kind of suspending shaft, a self-made rotating cantilever bending test machine shown in Fig. 2 was designed and produced. It should be pointed out that the shaft is usually subjected to a combination of torsion and bending loads. The shear stress due to torque always keep constant during the operation, but the bending load and bending stress due to the bending load are changing as the shaft rotates. The shear stress due to torsion only affects the mean value of fatigue stress, so more attention is paid on the bending stress and the shear stress is neglected. The motor rotate velocity of the equipment can be adjusted according to the ex-periment demands. In this research work, the fre-quency of 3.3 Hz (200 rpm) was used to simulate working conditions of a shaft and a non-symmetric cyclic stress was obtained via using a cam under the load level. 2.3. Test solution

The corrosive solution was collected from the ex-pander dryer directly. The analysis via ion analytical equipment indicated a pH level of 6.0 and Cl- content of 300 ppm, i.e. the shaft works in a corrosive envi-ronment. In the experiments, the corrosive solution was dropped into the notch from the corrosive solution dropping set while the specimen was subjected to a repeated bending load (refer to Fig. 2). This experi-ment method and equipment can simulate the real working condition of the shaft in the factory.

Table 1. Chemical composition of 17-4PH (wt %).

C Cu Ni Nb Cr Fe≦ 0.07 3.5-5.0 3.0-5.0 0.45-0.5 15.7-17.51 Bal

Table 2. Chemical composition of 1Cr13 (wt %). C Si Mn P S Cr Fe≦

0.15 ≦ 1.0

≦ 1.0

≦ 0.035

≦ 0.030 11.5-13.5 Bal

Table 3. Chemical composition of 2Cr13 (wt %).

C Si Mn P S Cr Fe0.16- 0.25

≦ 1.00

≦ 1.00

≦ 0.035

≦ 0.030 12.0-14.0 Bal

Fig. 1. Specimen configuration used in fatigue test.

Fig. 2. Schematic diagram of self-made rotating cantilever bending test machine.

3. Results and discussions 3.1. Material mechanical properties

Before fatigue experiments, the tensile tests were performed to obtain the mechanical characteristics of the materials, the results are referred to Table 4. 3.2. Microstructure of fatigue cracks

Fatigue tests were performed in both laboratory air and an aggressive environments. For 17-4PH, after subjecting to a cycle number of more than 2.68×106, fatigue crack was observed near the notch in laboratory air environment. However, in cases of 1Cr13 and 2Cr13, the cracks were observed when cycle numbers were less than 1.0×106. The experimental data shows that 17-4PH is better than 1Cr13 and 2Cr13 in resis-tance to fatigue damage in the air environment.

When the fatigue tests were performed in chloride corrosive solution, the fatigue cracks appeared in al-most the same cycle number among the three cases. This means that the corrosive environment had less effect on the fatigue crack propagating.

Table 4. The mechanical properties of the materials.

Materials 17-4PH 1Cr13 2Cr13Young’s modulus (GPa) 191 198 213

Yield strength (MPa) 912 712 766 Tensile strength (MPa) 963 829 874

Elongation rate (%) 12 17 18.8

Journal of Pressure Equipment and Systems 3 (2005) 157-161 159

Fig. 4 shows an example of crack surface morphol-ogy of 17-4PH observed via scanning electron micro-scope (SEM) after experiment. Fatigue veins and cor-rosion pits can be clearly observed on the fatigue crack surface.

Figs. 5(a) and 5(b) show SEM observed fatigue cracks in air and corrosive environments respectively. Compared to only one fatigue crack in laboratory air environment, secondary fatigue cracks were observed in corrosive fatigue test, as shown clearly in Fig. 5(b). This indicated that the corrosion fatigue damage had the characteristics of branch cracks.

From Fig. 5(b) we can also find many surface pits, which were caused by corrosive solution soon after the start of experiment. Pits sometimes are isolated but often clustered together to form a roughened surface. Generally, they act as stress raisers facilitating the ini-tiation of fatigue cracks [9, 11, 12]. 4. Crack growth rate

Fig. 4. Corrosive fatigue pits at the surface of crack 500X.

Fig. 5(a). A single fatigue crack at air environment 500X.

Fig. 5(b). Branched fatigue crack in corrosive environment. 4.1. The lg (da/dN)-lg(K) curve

Paris equation describes the relation of da/dN and ΔK as follows [13]:

mKCNa

Δ= 1dd (1)

where a is crack depth, N is cycle number, ΔK is stress strength factor, C1 is a constant.

The above may be rearranged as

)lg(ddlg 1

mKCNa

Δ= (2)

a and N can be measured and recorded in the test process. The stress strength factor can be calculated by the following equation.

( )πK Y D aσΔ = Δ + (3)

where Y - shape factor D - crack open displacement a - crack length Δσ - stress amplatitude.

Shape factor Y can be calculated by the following

32

26 [1.99 (1 )(2.15 3.93 2.7 )]

(1 2 )(1 )Y α α α α α

α α

− − − +=

+ − (4)

where

Wa

=α (5)

W - width of specimen.

According to the equations above, a series of lgΔK

and dlgd

aN

can be calculated. The results can be de-

J. W. Hao et al. / Comparison of fatigue crack growth rates of 17-4PH, 1Cr13 and 2Cr13 in chloride environment 160

scribed in coordinates with lgΔK as x-axis and dlgd

aN

as y-axis. The crack growth rate is obtained from the regression slope of a log-log power law plot.

4.2. The test results of 17-4PH in air and corrosive environment

Fig. 6 describes the crack growth rates with the comparison of that in lab air environment with that in chloride corrosive solution. From Fig. 6 it can be dis-covered that the crack growth rate in Cl- corrosive so-lution is slower than that in laboratory air environment.

It seems to contradict to our conventional knowl-edge that the fatigue crack growth would be sped up due to the effect of corrosive solution. Generally, when the fatigue damages happened in corrosive environ-ment, the crack surface is always covered by a layer of corrosion substances. The existence of the corrosion substances prevents the cracks returning to its original shape even the applied load is relieved, therefore this reduces the effective stress amplitude at the top of cracks. As explained in the preceding section, many pits were caused on the crack surface when corrosion fatigue happened. During the formation of pits, many corrosion substances were produced and which couldn’t be expelled out the crack timely. Therefore it possibly results in the difficult of crack closure and blunts the crack growth rate. Compared to the linear single fatigue crack in laboratory air, the corrosion fa-tigue cracks have a branching shape as the develop-ment of fatigue damage. Due to multiple cracks, the stress intensity factor at the top of cracks is released during the propagation of fatigue cracks. Consequently this also delayed the crack growth rate efficiently in corrosive environment.

Figs. 7 and 8 describe the fatigue crack growth rates of 1Cr13 and 2Cr13, respectively. As shown in Figs. 7 and 8, the crack growth rates in the corrosive environ-ment in both cases are also less than that in air envi-ronment, just the same as the case of 17-4PH.

4.3. Comparison of crack growth rates among the three materials

As shown in the preceding sections, the crack growth rates in the corrosive environment in three cases become slower than that in air environment. Compared the fatigue crack growth rates among the three materials, we can see that the crack growth rates in Cl- corrosive environment are different from each other. In case of material 1Cr13, the fatigue crack growth rate is the fastest among three materials in cor-rosive environment. The fatigue crack growth rates in cases of 17-4PH and 2Cr13 are almost the same, both show a slower growth rate than that of 1Cr13. How-ever, the price of 2Cr13 is cheaper than that of 17-4PH, and the material 2Cr13 also has a easy mechanical processing. Therefore, the material 2Cr13 can be cho-

sen as an alternative material working in corrosive en-vironment. In fact, the shaft made of 2Cr13 has been in service for two years, and it is still in a good condition.

Fig. 6. Crack growth rates of 17－4PH in air and corrosive environment.

Fig. 7. Crack growth rates of 1Cr13 in air and corrosive envi-ronment.

Fig. 8. Crack growth rates of 2Cr13 in air and corrosive envi-ronment.

Journal of Pressure Equipment and Systems 3 (2005) 157-161 161

5. Conclusion

In this study, in order to find out an alternative ma-terial which can be used to make shafts working in corrosive environment to replace 17-4PH, corrosion fatigue tests were performed on three different kinds of materials of 17-4PH, 1Cr13 and 2Cr13. The following conclusions were obtained.

(1) The corrosion fatigue cracks are multiple and branched, while the fatigue crack in air environment is single.

(2) The fatigue crack growth rate is delayed in chlo-ride corrosive solution when subjected to a nonsym-metric cyclic stresses (r = 0). This is considered to be caused by: (a) the corrosion substances produced dur-ing the formation of corrosion pits result in the crack closure difficulty; and (b) the multiple cracks which releasing the stress intensity factor also blunt the crack growth rate.

(3) Stainless steel 2Cr13 can be chosen as an alter-native material working effectively in corrosive envi-ronment. Practical application seems quite promising.

Acknowledgement

We are grateful for the financial support of this

study provided by the Maoming Branch of China Pe-troleum & Chemical Corporation (Sinopec Corp.) and their help for the preparation of corrosive solution. The authors would also like to thank Mr. X. Luo (a under-graduate student at East China University of Science and Technology) for his interest and assistance with the work.

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